A wireless sensor network (WSN) is a special case of wireless networks, the specific characteristics thereof including generally very strict requirements for minimizing the physical size and power consumption of node devices, as well as specialized roles for the nodes: a large majority of the nodes are sensors that collect information and convey it to certain data sink nodes, which are few in number and may act as gateways that pass the collected information to other networks and systems. Wireless sensor networks have frequently an ad-hoc nature, which means that nodes may come and go or roam from one part of the network to another, and the network must adapt itself to the consequent changes in topology and connectivity, often necessitating multi-hop routing capability. Throughput rates in sensor networks are usually relatively low, at least compared with the Mbit/s level data rates of communications networks between computers. For the sake of generality also actuators may be considered as nodes; it is conventional to understand the word “sensor” in wireless sensor networks widely to encompass both true sensors and actuators.
Wireless sensor networks and their nodes are known in general from numerous prior art publications. A publication US 2004/0100917 A1 discloses a coordinator device election process, the purpose of which is to ensure that no part of an ad-hoc wireless sensor network becomes disconnected, as well as to minimize the overall amount of energy needed for setting up arranged communications through the network. The solution disclosed therein is based on distributing an initialization message through the network, after which each node applies a random delay before broadcasting a “request for coordinator status” message. Another publication US 2003/0152041 A1 describes certain general level features of wireless sensor networks, including hierarchical allocation of nodes to a plurality of node levels as well as keeping a node dormant at all other times than initializing the node or making it perform a task.
A prior art publication US 2002/0044533 A1 mentions the drawbacks of depending on the accurately known spatial position of each node, which conventionally required each node to comprise a GPS (Global Positioning System) receiver. As a more advantageous alternative said publication presents a system in which each node finds out the full set of other nodes it could basically communicate with, but only maintains active communications with a subset thereof, which leads to a position independent way of setting up and maintaining network topology. Another prior art publication CA 2 311 245 A1 considers the division of nodes into two hierarchical levels, so that each higher-level node governs a cluster of neighboring lower-level nodes, and higher power “trunk line” communications are only needed between the higher-level nodes.
A prior art publication WO 01/69279 discloses a wireless sensor network in which each node has its own locating device, and the nodes are capable of exchanging both location information and reconnaissance data. The system is mainly meant for military reconnaissance purposes. Another prior art publication WO 01/26329, which is a member of a very large family of interrelated patent applications, discloses a very large number of details that at the time of writing this description are already considered to form the generally known state of the art of wireless sensor networks. Yet another prior art publication is U.S. Pat. No. 6,208,247 B1, which focuses mainly on the physical implementation of node devices for wireless sensor networks.
The main source of difficulties on the route towards minimized power consumption in wireless sensor networks is traditionally the communications protocol that determines the amount and nature of wireless transmissions between the nodes. The fact that the network must be able to dynamically adapt to appearance and disappearance of nodes as well as other changes in network topology means that the communications protocol must include sufficient procedures for discovering currently available possibilities of communicating with other nodes, as well as routines for determining the order in which the currently connected nodes communicate with each other. The communications protocol should involve a certain degree of scalability, which means that it should facilitate energy efficient communications regardless of how many nodes there are in the network. Additionally the communications protocol should ensure some required minimum level of throughput, i.e. amount of information that can be transmitted to a desired destination through the network in some unit of time. The most important part of the communications protocol is believed to be the MAC part (Medium Access Control).
Known protocols for wireless sensor networks include the Sensor-MAC (also known as S-MAC), the Timeout-MAC (T-MAC) and the IEEE 802.15.4 Low Rate Wireless Personal Area Network (LR-WPAN) standard. Of these, the S-MAC has been described in the scientific publication W. Ye, J. Heidemann, and D. Estrin: “Medium access control with coordinated, adaptive sleeping for wireless sensor networks,” ACM/IEEE Trans. Networking, vol. 12, no. 3, pp. 493-506, June 2004. It utilizes a common slot structure within a Carrier Sense Multiple Access (CSMA) MAC scheme. Nodes are scheduled to be periodically awake and sleep, which reduces significantly energy consumption compared to conventional CSMA. An S-MAC slot consists of a short beacon type synchronization transmission, a fixed length (300 ms) active period for data exchange, and a sleep time until the end of the slot. Each node wakes up at the beginning of a slot and any node wishing to transmit data performs CSMA/CA channel access with an RTS/CTS (Request To Send/Clear To Send) handshake. The slot length is a predefined and static MAC parameter in the order of 500 ms to 10 s. In the newest implementation, S-MAC protocol performs 10 s long network scanning every 2 minutes. Clearly, network scanning consumes a high amount of energy.
The Timeout-MAC (T-MAC) is described in the scientific publication XXX. The protocol is similar to S-MAC, but energy efficiency is improved by adjusting dynamically the length of the active period. A node goes to sleep mode if it cannot receive any activity on the channel within a 15 ms time-out interval. In contrast to S-MAC, slot length is fixed to 610 ms. T-MAC performs sporadically 610 ms long network scanning. This short scanning time keeps energy consumption quite low.
The IEEE 802.15.4 LR-WPAN standard is a member of the IEEE 802.15.X family of WPAN standards and has been described in “Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks (LR-WPANS)”, IEEE Std 802.15.4-2003 edition. It utilizes CSMA/CA channel access with optional use of superframe structure, which is quite similar to S-MAC slot structure. A LR-WPAN network consists of coordinators, which provide synchronization services and route data in a network, and devices, which may only communicate with coordinators. Coordinators control their superframe structures by transmitting beacons at the beginning of each superframe. A beacon is followed by a Contention Access Period (CAP), during which network nodes can transmit data and requests to a coordinator using CSMA/CA. Optional dedicated assignment type Guaranteed Time Slots (GTS) after CAP reduce contention and latency. The CAP and GTS are optionally followed by an inactive period until a next beacon during which nodes may go to sleep mode. Beacon period and superframe length are variable between 15.4 ms to 252 s, which enables a trade-off between latency and energy consumption. LR-WPAN nodes perform network scanning periodically over a set of RF channels. Each channel is received for a beacon interval or until a specified number of beacons has been received. The worst case scanning time is over 67 minutes, when the maximum beacon interval and all 16 channels specified for the 2.4 GHz frequency band are scanned. This equals the energy of several million beacon transmissions. Clearly, the consumed energy for network scanning may be very high.
A further development to the above described LR-WPAN is known as the ZigBee and described online on the official website of the ZigBee alliance (http://www.zigbee.org). It includes network and security layer definitions and application profiles, and supports access control lists, packet freshness timers and certain encryption standards.
Another protocol suggestion is the Self-Organizing Medium Access Control for Sensor Networks (SMACS), described in the scientific publication K. Sohrabi, J. Gao, V. Ailawadhi, and G. J. Pottie: “Protocols for self-organization of a wireless sensor network,” IEEE Personal Communications, vol. 7, no. 5, pp. 16-27, October 2000. It enables nodes to discover their neighbours, form links, and establish schedules for transmission and reception without the need of master nodes. A network uses peer-to-peer topology with Frequency Division Multiple Access (FDMA), where each link operates at different RF channels. SMACS protocol is based on stationary wireless nodes. However, the protocol has an extension for mobility management by an Eavesdrop-And-Register (EAR) algorithm, which enables interconnection of mobile nodes in the field of stationary wireless nodes. SMACS enables quite high energy efficiency due to scheduled data exchange slots. The disadvantages are high performance requirements for each network node and the support for only limited mobility.
Linked Cluster Architecture (LCA) is a solution that has been known already for over twenty years. It was first described in the scientific publication D. Baker, and A. Ephremides: “The architectural organization of a mobile radio network via a distributed algorithm,” IEEE Trans. Communications, vol. 29 no. 11, pp. 1694-1701, November 1981. It improves scalability by organizing the network into a set of clusters, each having a cluster head, which acts as a local controller. Other nodes are either ordinary nodes, or gateway nodes, which both are in direct communication range with the cluster head. LCA utilizes Time Division Multiple Access (TDMA) MAC with dedicated time slots for each node. The regular data transfer is suspended periodically by a control phase to perform a distributed clustering algorithm, during which neighboring nodes are detected and logical node function chosen. In LCA, nodes record and maintain information about their immediate environment, which makes the protocol quite well scalable. The disadvantage is that LCA utilizes a global TDMA frame structure, which requires a global clock. Additionally since the nodes are only aware of their neighboring clusters, multi-hop routing is not supported.
Yet another known protocol is the Low-Energy Adaptive Clustering Hierarchy (LEACH) described in the scientific publication W. Heinzelman, A. Chandrakasan, and H. Balakrishnan: “An application-specific protocol architecture for wireless microsensor networks,” IEEE Trans. Wireless Communications, vol. 1, no. 4, pp. 660-670, October 2002. It is a dedicated assignment MAC protocol with clustered topology. LEACH extends the network hierarchy by a base station, which acts like a root in a network. Cluster heads and the base station employ only direct communications. Thus, a star topology is utilized in two hierarchical levels. LEACH protocol improves total network energy efficiency by allowing most nodes to transmit short distances, and requiring only cluster heads to use high transmission power for communicating with a base station. Consequently, the cluster heads have a shorter battery life than other nodes, which may reduce the overall network lifetime. To distribute energy consumption more evenly, LEACH proposes to rotate cluster heads randomly. A drawback is that the network scalability is limited by the coverage and the performance of the base station.
All known protocol suggestions include drawbacks that make the unsuitable for use as the communications protocol for the ultimate low-power wireless sensor network. Many of them simply require too many transmissions to qualify as truly low-power solutions. In others, the time a node typically needs for network scanning is prohibitively long. One specific problem that arise in many otherwise advanced protocols is that they require using rather complex hardware at each node, for example for repetitively measuring and storing multiple RSSI (Received Signal Strength Indicator) values.